Congenital cardiovascular malformations are the leading cause of neonatal and infant death and a major cause of adult cardiac insufficiency. Valvular stenosis and hypoplastic left heart syndrome are the most common malformations occurring in eight out of 1000 live births and constituting 25-30% of all cases of human cardiovascular malformations. There is a critical need to understand the mechanisms that cause these diseases. Our long-term goal is to identify the mechanisms and causes of these cardiac diseases and to develop tools to prevent their incidence. The objective of these studies is to determine whether an association between genetic and environmental factors is the ultimate cause responsible for disease incidence. Our central hypothesis is that prenatal exposure of mice to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin), an organochlorinated environmental pollutant, mediates the aryl hydrocarbon receptor (AHR)-dependent repression of the homeobox transcription factor NKX2.5, a key determinant of cardiac morphogenesis. Our rationale is that mutations in the NKX2.5 gene and maternal exposure to halogenated hydrocarbons, dioxins and polychlorinated biphenyls during pregnancy are the main known risk factors for these human developmental cardiac malformations. Key in this context is our finding that AHR represses NKX2.5 expression. Completion of our specific aims will achieve the following short-term goals: (1), to characterize the AHR role in cardiovascular development and NKX2.5 regulation; (2), to identify regulatory gene expression changes during cardiovascular development that result from TCDD exposure; (3) to determine if TCDD exposure during embryonic development causes long-lasting cardiovascular malformations. These aims test the working hypothesis that organochlorinated environmental agents disrupt AHR-regulated functions during cardiac embryonic development and cause long-lasting cardiovascular injury. We will use promoter-mediated selection methods to model the gene-environment interactions established between NKX2.5, AHR and dioxin exposure and use global gene expression analyses and chromatin immunoprecipitation to identify genetic and epigenetic signatures characteristic of TCDD effects during differentiation. We will probe tissues of newborn and adult mice exposed to TCDD in utero for the expression of target gene clusters, epigenetic changes and incidence of cardiac malformations. The significance of this work lies on the identification of the molecular mechanisms by which AHR and TCDD exert their cardiac effects, providing a test of the causal connections between AHR, dioxin and cardiovascular disease. The novel approach that we propose will define a path applicable to many similar studies of other environmental agents. Our work is innovative because it proposes to use a combination of molecular tools and ES cell research never before employed in the study of the mechanisms of action of environmental agents. We expect to establish a strong mechanistic link between TCDD exposure, the genes that the AHR activates and cardiac malformations.